FIG. 12 is a block diagram showing the configuration of an optical characteristic measuring apparatus according to the prior art. As shown in FIG. 12, the measuring system is divided into a light source system 10 and a characteristic measurement system 20. A variable wavelength light source 12 of the light source system 10 varies the wavelength to generate a light (variable wavelength light) having wavelengths of λi and λi+1. The variable wavelength light will be modulated by a light modulator 14. The light modulator 14 includes LN (lithium niobate). The light modulator 14 receives an electrical signal having a frequency of fi from a modulation power supply 16 and modulates the variable wavelength light with the frequency fi.
The light outputted from the light modulator 14 is introduced into an optical fiber or other DUT (device under test) 30. The transmitted light having transmitted the DUT 30 will be supplied to an optical/electrical converter 22 of the characteristic measuring system 20. The optical/electrical converter 22 proceeds to an optical/electrical conversion of the transmitted light and outputs to a phase comparator 24. The phase comparator 24 measures the phase of the output signal of the optical/electrical converter 22 with reference to the electrical signal produced by the modulation power supply 16. Here, the phase when the incident light wavelength is λi will be represented by φi and the phase when the incident light wavelength is λi+1 will be represented by φi+1. The characteristic computing section 26 will compute the wavelength dispersion characteristic and other characteristics of the DUT 30 from φi and φi+1.
The operation of the characteristic computing section 26 will be described with reference to the phase-wavelength diagram shown in FIG. 13. When φi+1−φi is represented by Δφ, the group delay time is computed from Δφ and the modulation frequency fi, and then the wavelength dispersion is computed therefrom.
Here, the range of phase difference that can be measured from the phase comparator 24 extends from −π to π. Therefore, it is preferable that φi+1−φi would be within the range extending from −π to π. This is because any large modulation frequency fi can easily exceed the range of −π to π. In other words, when the same time difference is expressed by the phase difference, the bigger the frequency is, the cycle is shorter, and when it is expressed by the phase difference, the cycle will be longer. For example, when the time difference is 1/50 secs., if the frequency is 1 Hz, the range of phase difference is only 0.04π, but if the frequency is 50 Hz, it will be 2π. Therefore, the modulation frequency fi should be lowered to the minimum possible, and the wavelength λ of the incident light should be varied.
However, in order to measure Δφ with a high precision, it is preferable that the modulation frequency fi has a high value. This is due to the fact that, when a same time difference is expressed with a phase difference, the larger the frequency is, the shorter the cycle becomes, and when it is expressed with a phased difference, the cycle will be greater.
Therefore, the present invention has an object of providing devices enabling to enlarge the modulation frequency that modulates a variable length wavelength generated by the light source without making problem with respect to the measurement of the optical characteristic.